Eventually, it will happen. It's just building up towards it with their relationship. It's not just about the destination but also to enjoy the journey with this game.
Not exactly catching her but he has heard her masturbating depending on the player's choices.
When she is watching tv and then switches to porn and starts masturbating the player can decide if Sophia keeps watching and masturbating which will lead to Sophia getting caught by Ellie
OR change the channel and Ellie won't catch her and instead Sophia will go and take a shower and continue touching herself and moaning and then she'll be heard by Dylan and he'll ask if she is okay so, it's not much about catching her in plain sight but Dylan would most likely know what was she doing in the shower because of the moans.
I wonder if that will play out later in the game or if it's just an isolated moment.